An iron phosphate hydroxide hydrate electrocatalyst: synergistic effects of Fe2+ and Fe3+ for enhanced hydrogen evolution reaction stability

Abstract

Transition metal phosphates (TMPs) have evolved as one of the highly promising electrocatalysts for dissociating water due to their remarkable catalytic qualities. Herein we have successfully synthesized iron phosphate hydroxide hydrate on a nickel foam substrate by an electrodeposition method and the molar ratio was optimized to achieve the lowest possible overpotential required to form hydrogen gas. The proton acceptor (negatively charged phosphate (P) sites) and hydride acceptor (coordinated metal sites) promote proton adsorption at the electrocatalyst surface during water electrolysis and enhance the performance. Despite this advantage, phosphate materials are not stable over a period. Therefore, to prevent the degradation and material dissolution, a particular composition of Fe³⁺ and Fe²⁺ is obtained to suppress the oxidation of the material. The catalyst Fe₃(PO₄)(OH)₆·3H₂O exhibits the highest mass activity with a tuned Fe³⁺/Fe²⁺ ratio and resulted in a lower overpotential of 130 mV at 10 mA cm⁻² for the hydrogen evolution process. Furthermore, this catalyst is electrocatalytically stable for 80 h at 100 mA cm⁻² current density with low activity degradation.